Myriad / myriad of

So i’m a PA & I’ve been having an argument with my boss over the word myriad.

I was under the impression that it stands alone: “there were myriad apples on the fruit-seller’s stall” but he argues that it is correct to say “there was a myriad of apples on the fruit seller’s stall”

Both should be correct. One is used as a noun, the other used as an adjective. But what I'm not sure about is if there are any differences in nuance. For instance, saying "many apples" and "many of the apples" are quite different in meaning; the former means a lot of apples and the latter is a subset of a larger pool of apples.

"A 10,000 of apples" is indeed poor English; however, as was pointed out, the <i>original</i> meaning of "myriad" was 10,000. That's no longer the common meaning. Thus, "a myriad apples" and "a myriad of apples" are both correct. Normally, though, when we're counting apples, we use pounds, kilograms, bushels, or simply, "a lot" to describe a large number.